Truly, spam isn't a technical problem, it's a social one. If we could figure out some way to make mail recipient networks and hosts willing to shun known bad actors, even at the cost of losing some real mail for a while until the bad actors cave, it would make vastly more difference than any possible technical changes. No matter how super secure an improved protocol is, if a mail system operator lets people send spam through it, the recipients still have to decide how they're going to deal with the sender's behavior.

The reason that e-mail is uniquely useful is that you can exchange mail with people you don't already know. The reason that spam exists is that you can exchange mail with people you don't already know.